The operation of an internal combustion engine such as, for example, a diesel, gasoline, or gaseous fuel-powered engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of the engine piston forces exhaust gas, which may include these emissions, from the combustion chamber to the atmosphere.
Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of emissions emitted from an engine to the atmosphere may be regulated depending on the type of engine, size of engine, and/or class of engine. One method implemented by engine manufacturers to comply with the regulation of exhaust emissions is described in U.S. Pat. No. 4,768,475 (the '475 patent) issued to Ikemura on Sep. 16, 1988. The '475 patent describes varying the typically fixed valve timing of an engine in order to improve the combustion process. Specifically, the '475 patent discloses a valve mechanism having a first cam and a second cam. Each of the first and second cams has a common base circle, but the lobe lift amounts and opening/closing angles are different. A first rocker arm engages the first cam, and a second rocker arm engages the second cam. The rocker arms have holes, which three pins engage to selectively connect the first and second rocker arms with an actuating arm so as to be rocked together with the actuating arm. During a low engine speed condition, the pins are moved to engage the first rocker arm with the actuating arm. During a high engine speed condition, the pins are moved to engage the second rocker arm with the actuating arm. In this manner, the first cam having a low valve lift and a small opening angle is selected to control motion of an intake valve during low engine speeds, and the second cam having a high valve lift and a large opening angle is selected to control motion of the intake valve during high engine speeds.
Although the valve mechanisms of the '475 patent may improve the combustion process by providing variable valve timing, it may be limited. In particular, because the '475 patent only provides two levels of valve actuation (e.g., two fixed valve lifts and two fixed opening/closing angles), it may not allow efficient combustion throughout the range of engine operation. In addition, because the valve mechanism of the '475 patent is only associated with the intake valve, some emission reducing methods such as, for example, internal exhaust gas recirculation, may be not be possible.
The engine valve actuation system and method of the present invention solves one or more of the problems set forth above.